I wish I had some good news for you, but I am in the same boat as you. I finally stopped taking all prescribed meds and use OTC IBS meds which, as you mentioned, sap all of the fluids from my body and (I'm sure) depletes my potassium.
We are darned if we do, darned if we don't. IBS is a constant struggle, weather you are dealing with constipation or diarrhea or a combination of both (as I am). Doctors do very little to really help and I just couldn't bear the drugs any longer. I may be doing more damage to myself, but so far my regimin is working for me!
Peace and Blessings